Positrons decaying in human tissue give rise to two oppositely travelling .gamma.-quanta which can be detected by two .gamma.-detectors disposed opposite one another across the human tissue and flanking the body portion examined.
The .gamma.-detectors are generally connected to a computer which, upon coincidence of the detection of two .gamma.-quanta from signals of the opposing .gamma.-detectors, can register a given positron decay. The computer can calculate the flight paths of the two .gamma.-quanta arising from the same positron decay and thus generate position information as to the decaying positron.
The position information can be stored in the computer and the positions of the .gamma.-detectors varied so that, from the sum total of the position information obtained, a distribution function of the positron decays can be generated and the computer can, from the distribution function, generate a density distribution of the positron decay which represents information relating to the tissue.
This process is referred to as positron emission tomography (PET) and is increasingly being employed for medical evaluations and in scientific research as to human tissues, pathological conditions and the like.
A patient is injected with or inhales a radio-pharmaceutical with a positron emitting isotope and the localization of the positrons in the human organism can be detected by the PET. In human tissue, the emitted positrons have a range of millimeters to several centimeters depending upon their energies which are typically between 1 and 5 MeV, and decay when they come to rest in the aforementioned manner.
In the present day PET scanners, individual detectors are assembled in opposing detector banks and are capable of detecting only a small part of the emitted radiation. It is thus required to rotate the detector assemblies around the object to be measured and to shift the detector assembly vertically to be able to cover a relatively large space angle in a sequence of measurements. This requires a relatively long measuring period for the process which an result in an undesirably long exposure of the human organism to radiation and thus an undesirably large radiation load on the patient.
Apart from such individual detectors, .gamma.-quanta can be measured by local resolution photomultipliers having scintillators ahead of photomultiplier tubes and at the exposure window. These devices can provide a detector surface with diameters of 10 cm and can achieve a local resolution of 1 mm.
Typically 10.sup.8 decay events can be detected in the course of a single measurement process to determine density distribution of the positron decay events. By the use of such photomultipliers instead of individual detectors, the detection of 10.sup.8 decays can be substantially accelerated and thus the period of measurement can be reduced by an order of magnitude. However, this usually requires 16 angular positions of the photomultipliers around the subject and four vertical positions so that the memory for the resulting data must be at least 425 MB (megabytes) to afford the data search required to provide the desired information as to the density distribution.
The computers usually used for positron emission tomography, however, generally are incapable of processing this substantial data flow from the photomultipliers in an on-line mode, nor do they have such high data storage capacities.